Today, PS (presensitized) plates in which positive type photosensitive agents of which the principal components are a diazo compound and a phenolic resin, or negative type photosensitive agents of which the principal components are acrylic based monomers or prepolymers, are being used for lithographic offset printing plates. These plates have low sensitivity. Plate making is carried out by attaching thereto a pre-imagewise recorded film original and exposing the plate to light.
On the other hand, as a result of processing which has been made in computer graphics processing and large capacity data storage coupled with data transmission techniques, in recent years, electronic editing systems, in which everything from original input, correction, editing and layout to page compilation is handled by computer and can be sent out immediately to terminal plotters in distant locations by means of a high speed communication network or by satellite communication, have been placed into practical use. The need for electronic editing systems is especially great in the newspaper printing field where immediacy is essential. Furthermore, as a result of the development of ultralarge capacity memory devices, such as optical disks, consideration has been-given to the storage of originals as digital data on these recording media in fields where the original is stored as a master film and a plurality of printing plates are made, as required, on the basis of the stored original.
However, practical direct type basic printing plates from which a printing plate can be made directly from the output of a terminal plotter are not available Even when an electronic editing system has been used, the output is materiallized using a silver salt based photographic film in practice and the printing plate is produced indirectly on the basis of this film which is attached to a PS plate which is then exposed. One of the reasons for this is the difficulty involved in the development of direct type printing plates which have a sufficiently high speed to enable the printing plate to be made in a practical period of time using the light sources of output plotters (for example, He--Ne lasers and semiconductor lasers).
Electrophotographic photoreceptors are highly sensitive to light and could provide direct type printing plates. Many printing plates for electrophotographic type plate making of the type in which the photoconductive layer in the non-image parts is removed after forming a toner image are well known. Thus, printing plates for electrophotographic type plate making have been disclosed, for example, in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426, JP-B-46-39405, JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-57-147656, and JP-A-57-161863. (The terms "JP-A" and "JP-B" as used herein mean "unexamined published Japanese patent application" and "examined Japanese patent publication" respectively.)
Because it is necessary to remove the non-image parts by etching to expose a hydrophilic surface in order to use an electrophotographic photoreceptor as a printing plate, binder resins which dissolve or swell in alkaline solvents and are then stripped away are often used as binder resins. Normally, these resins which dissolve or swell in alkaline solvents have poor compatibility with organic photoconductive compounds when compared to the polycarbonate resins which are widely used as binder resins for electrophotographic photoreceptors and, consequently, the amount of organic photoconductive compound which can be introduced into the electrophotographic photosensitive layer is limited. Even if sufficient amount of carriers for eliminating the surface potential is formed in the photoconductive layer, if the organic photoconductive compound content in the photoconductive layer is low, then the rate of migration of the carriers in the photoconductive layer is reduced and the rate at which the surface potential falls is low, which is to say the response rate is low. Consequently, after the completion of exposure, the time period from that the surface potential has been reduced sufficiently so that fogging does not occur to that toner development can commence is prolonged. The response time becomes longer as the exposure brightness is increased for a shortened exposure time in order to shorten the processing time as much as possible. Hence, the slowness of the response time greatly impedes any shortening of the overall processing time. Furthermore, other problems arise in cases where a scanning exposure is carried out using a high brightness light source such as laser light source. Thus, if the response rate is low, the rate of reduction of the surface potential differs in the parts where write-in starts and finishes, and so there is no fogging in the parts where write-in starts but fogged images are formed where write-in finishes and irregularity inevitably occur when a printing plate is made. Known binder resins which have been used in the past in printing plates for electrophotographic type plate making include styrene/maleic anhydride copolymers, vinyl acetate/crotonic acid copolymers, vinyl acetate/maleic anhydride copolymers and phenolic resins as disclosed, for example, in JP-B-41-2426, JP-B-37-17162, JP-B-38-6961, JP-A-52-2437, JP-A-54-19803, JP-A-54-134632, JP-A-55-105254, JP-A-50-19509 and JP-A-50-19510. However, various problems are known to arise when these are used in printing plates for electrophotographic plate making in which organic photoconductive compounds are used. Thus, the formed film is hard and cracking occurs when the printing plate is flexed in cases where a styrene/maleic anhydride copolymer has been used as a binder resin. Furthermore, film adhesion is poor and unable to withstand the printing of a large number of copies. The film which is formed is brittle and has poor printing durability when a phenolic resin has been used as a binder resin. Problems also arise with printing durability when a phenolic resin has been used as a binder resin. Problems also arise with printing durability when vinyl acetate/crotonic acid copolymers and vinyl acetate/maleic anhydride copolymers are used.
The use of copolymers of acrylic acid ester monomers or methacrylic acid monomers with monomers which contain a carboxylic acid has been suggested in JP-A-57-161863 and JP-A-58-76843 as a means of overcoming the various problems which result in poor printing durability, principally of the type described above. It is possible to use printing plates for electrophotographic type plate making if these resins are used, but the problems which have arisen in recent years due to the slow response rate, as mentioned earlier, have still not been resolved.